1. Field of the Invention
The present invention relates to a planar antenna array, and more particularly, to an antenna apparatus for a base station of a mobile communication system such as cellular (900 MHz), PCS (Personal Communication Services) (1800 MHz) and other wireless communication systems.
2. Description of the Related Art
Planer array antennas are known to come in various forms and have many different purposes. A few examples of such planer array antennas are provided, and incorporated herein by reference, by U.S. Pat. No. 5,061,943 to Emmanuel Rammos and entitled Planar Array Antenna, Comprising Coplanar Waveguide Printed Feed Lines Cooperating with Apertures In A Ground Plane; U.S. Pat. No. 5,307,075 to Tan D. Huynh and entitled Directional Microstrip Antenna With Stacked Planar Elements; and U.S. Pat. No. 5,841,401 to Martin R. Bodley et al. and entitled Printed Circuit Antenna. The 5,841,401 patent has use as a base station antenna in cellular and PCS systems.
An array of cylindrical dipoles used for an early base station antenna is well disclosed in Mobile Antenna Systems Handbook, Artech House, 1994, pp. 126-127, by K. Fujimoto and J. R. James. The base station antenna ofthis kind has disadvantages of high production cost, large size and heavy weight.
A printed-array technology makes it possible to construct very thin, light-weight and cost-reduced antennas. An exemplary application of the printed-circuit technology to the base station antenna is presented in Broadband Patch Antennas, Artech House, 1995, by Jean-Francois Zurcher and Fred E. Gardiol. This base station antenna is a vertical linear array with vertical polarization consisting of so-called Strip-Slot-Foam-Inverted Patch (SSFIP) radiators. Functionally, an antenna consists of a microstrip power divider and square patch radiators electromagnetically coupled with it. The patches are coupled to microstrip feed line throw slots, etched in the ground plane of a microstrip line. A foam dielectric layer between the slot and the patch increases the antenna bandwidth. When the antenna is assembled in the sandwich form, it has a lightweight and resistant structure (of a composite material). Mechanically, the antenna has a multilayer structure consisting of a metal ground plate (a.k.a. ground plane), a first printed circuit board (PCB) with a microstrip divider and slots, a foam layer, and a second PCB with patches.
Although the printed base station antenna disclosed in Broad band Patch Antennas is cheaper in comparison with the early cylindrical dipoles, the cost of this antenna is still too high, because the PCBs of the antenna are made from high quality dielectric material to provide low insertion loss in the microstrip power divider. However, even if the high quality and high cost PCBs are used, the insertion loss in microstrip lines may be significant in the electrically big arrays, especially in the high frequency (1.8-2.5 GHz) PCS antenna. With use of this technology, the SSFIP antennas are acceptable for a medium gain (of about 13 dB), but it is still difficult to obtain a high gain (of about 16-20 dB). For example, the insertion loss and gain are 1.5 dB and 12.5 dB, respectively, for the antenna disclosed in Broadband Patch Antennas, despite using a high cost RT/duroid 5870 material for the PCB. Further, for the antenna of a gain 14 dB made by the same technology with the same PCB material, the insertion loss would be about 3 dB, because of an increase by about two times in length of the microstrip line. In the high gain (15-20 dB) antennas, the antenna efficiency is too low for the conventional technology.
Accordingly, the main technical problems of conventional technology are the high production cost and the significant insertion loss. It should be noted that the cost is very important factor for base station antenna because it is a requisite for mass production.